Activated mast cells and T lymphocytes, critical to the development of allergic inflammation, reside in close apposition at inflammatory sites. Potential effects of lymphocytes on mast cell degranulation and cytokine release were examined by co-culture. Human or murine mast cells co-cultured with activated T lymphocytes released histamine, beta-hexosaminidase, and TNF-alpha, an effect that peaked at twenty hours and which required direct cell-cell contact. Addition of the phosphoinositide 3-phosphate kinase (PI3 kinase) inhibitor wortmannin abolished the heterotypic adhesion-associated degranulation but not TNF-alpha production. Introduction of specific anti-adhesion antibodies (anti-LFA1 and anti-ICAM1) into the co-culture of murine mast cells and activated T cells inhibited mast cell degranulation. Moreover, FceR1-mediated exocytosis was augmented when primary mast cells were co-cultured with stimulated T cells, and this effect required direct cell-cell contact. The src family protein tyrosine kinase lyn plays an important role in mediator release induced by FceR1 crosslinking. Lyn has been shown previously to interact with specific tyrosine residues on the intracellular portion of FceR1 and induce their phosphorylation. Potential lyn interacting proteins were identified using yeast two hybrid screening. Mast cells contain numerous receptors coupled to heterotrimeric G proteins, including the anaphylatoxins C3a and C5a, which are known to generate inflammation. Regulators of G protein signaling proteins (RGS proteins) are GTPase activating proteins for heterotrimeric G protein alpha subunits and negatively regulate these signaling pathways. A novel role of RGS proteins was identified in the regulation of intracellular secretory transport through the interaction with a coatomer (COP1) subunit, Beta'COP. COP1 is a multi-unit protein complex that is thought to be required for maintenance of Golgi structure and formation of some secretory vesicles.